As disclosed in Japanese Laid-Open Utility Model Publication No. 4-109465, it has been well known that a fuel pump and a fuel filter for filtering impurities from fuel delivered from a fuel tank by the fuel pump can be integrally formed to be inserted into the fuel tank.
Further, it also has been well known that a pump module constructed by assembling a fuel pump and a fuel filter together can be inserted into a fuel tank.
When the fuel passes through the fuel filter, a surface of a filter case becomes electrically charged due to frictional electrification between the fuel and the fuel filter and due to fluid electrification between the fuel and the filter case. The magnitude of the electric charge or electrifying potential of the fuel case is proportional to the fuel flow rate through the fuel filter.
The electric charge does not readily build up on the surface of the filter case when the filter case is immersed in the fuel reservoir in the fuel tank. However, electric charge tends to readily build up as the amount of fuel remaining in the tank becomes less.
When electric charge builds up on the surface of the filter case, there is a possibility of generating a spark between the fuel tank and the filter case. This possibility becomes high when the pump module is removed from the fuel tank for repair work.
In the conventional arts, as disclosed in the Japanese Publication No. 6-500373, a conductive path is provided between the filter case and a vehicle body in order to prevent electric charge from building up on the surface of the filter case. However, when this technique is used for a fuel filter that is inserted into the fuel tank, provision of the conductive path between the filter case and the fuel tank complicates the device construction and decreases assembling efficiency.
On the other hand, in a conventional design disclosed in Japanese Laid-Open Utility Model Publication No. 4-109465, the discharge port of a return pipe is connected to a pressure regulator which is mounted in a fuel tank so that fuel returning from the pressure regulator may be dropped into the fuel reservoir from the discharge port of the return pipe.
The return fuel has kinetic energy when being dropped onto the surface of the fuel reservoir, and fuel return sound waves are generated upon impact.
The magnitude of the fuel return sound waves increases as the distance between the discharge port of the return pipe and the surface of the fuel reservoir increases, which results when the liquid surface lowers. This sound wave is transmitted through the fuel tank to the outside as noise.
Transmission of the fuel return sound waves to the outside can be reduced by constructing the fuel tank from a material having good sound insulating properties or by covering the fuel tank with a sound insulating material. However, these measures increase manufacturing costs and decrease assembling efficiency.
In order to solve the above problem, various studies have been made. As a result, the inventors of the present invention have found that electric charge generated on the surface of the filter case can be decreased by directing the return fuel to flow along a side of the filter case. In addition, the inventors have found that return sound waves can be reduced by this design.
Accordingly, an object of the present invention is to provide a fuel filter having a simple construction in which electric charge buildup on the surface of the filter case can be avoided at low cost, and in which return sound waves also can be reduced.